Despite human laboratory evidence documenting the reductions in cigarette craving, withdrawal symptoms and negative mood after an acute bout of physical activity, the efficacy of physical activity as a treatment for smoking cessation remains unresolved. Rather than conclude that physical activity is a weak or ineffective smoking cessation treatment, we hypothesize that there may be individual differences in the response to physical activity that render it an effective smoking cessation treatment for some smokers, but not at all effective for others. Behavioral Economic Theory (BET) may help identify those smokers for whom physical activity will have the strongest beneficial effects on smoking behavior. BET suggests that the reinforcing value of one behavior can be enhanced, or reduced, based on the alternatives. A substitute reinforcer (e.g., physical activity) can reduce the reinforcing value of an alternative (e.g., smoking) thereby decreasing the use of that reinforcer. However, physical activity may not function as a substitute reinforcer for smoking for all smokers. The goal of the proposed study is to evaluate whether individual differences in the relative reinforcing value of physical activity (RRVPA) moderate the effects of physical activity on the primary outcomes of craving, withdrawal, negative mood, positive mood, affective valence, and smoking reinforcement following brief abstinence. Hypotheses will be tested using a within-subject, 2-session (physical activity vs rest), behavioral pharmacology paradigm. Young adult smokers (n=78), ages 18-26 years old, will be recruited as they have the highest smoking prevalence of any adult age group and are less likely to succeed at quitting smoking compared to older adults despite more quit attempts. We will also explore the role of physical activity liking on the primary outcomes, whether negative and/or positive reinforcement processes are important to the RRVPA, and the distinguishing features of substitutability. This study will increase our understanding of who may benefit most from physical activity as smoking cessation treatment component and translate to and help inform effective smoking cessation interventions for young adult smokers.